(1) Field of the Invention
The present invention relates to a paper feeder for use in an image forming apparatus such as a copier, laser printer, etc.
(2) Description of the Prior Art
In a prior art typical sheet-conveyer used, for example, in copiers, when sheets are to be conveyed from a storage portion to a photoreceptor, the sheet is initially delivered to a registration roller by the rotation of feed roller and conveying roller. Then, after the feed roller and conveying roller stop rotating, the sheet is delivered toward the photoreceptor by rotating the registration roller and conveying roller in time with the rotation of the photoreceptor. At this operation, each of these rollers is rotated at the same peripheral velocity through gears and clutches by a driving force supplied by a motor.
Recent demands for high-speed operating copiers require sheet-feeding speed to increase, but it has been impossible to raise the copy speed in excess of a certain level because each roller should rotate at the peripheral velocity corresponding to that of the photoreceptor.
To deal with this, known prior art methods would take a measure in which only the operation during which the feed roller and conveying roller rotate may speed up. In this method, since the rotational peripheral velocity of the feed roller need be greater than that of the registration roller, the conveying roller is to include a speed-changing mechanism so as to vary its rotational peripheral velocity in dependence upon peripheral velocity of each roller. In such a paper conveying device equipped with the speed-changing mechanism, a driving force given by a motor is transmitted as shown in FIG. 1 from a driving gear 1 through gears 2, 3, 4, 5 and 6 to feed roller 7, conveying roller 8 and registration roller 9.
In this arrangement, as copying operation is activated, a driving clutch 10 is coupled so that rotation of the gear 6 may be transmitted to a shaft 7a thus rotating the feed roller 7.
At the time, when a speed-changing clutch 12 in a speed-changing mechanism 11 is connected, the gears 5 and 13 are coupled so that rotating force of the gear 5 is transmitted to a gear 13. The gear 13 causes a gear 14 to rotate which is integrally formed with a shaft 8a of the conveying roller 8, whereby the conveying roller 8 may be rotated. Since the gear ratio of the gear 6 to the gear 14 is set up as unit or one, the feed roller 7 and conveying roller 8 rotate at the same peripheral velocity. Accordingly, sheets are conveyed by the rotation in a direction shown in arrows A and B.
Subsequently, when a driving clutch 15 is connected in order to rotate the registration roller 9, rotation of the gear 2 is transmitted to drive the registration roller 9. At this time, by connecting a speed-changing clutch 16 simultaneously, rotation of gear 4 is transmitted to the conveying roller 8. As the gear ratio between the gears 2 and 4 are set up as one, the registration roller 9 and conveying roller 8 are adapted to rotate at the same peripheral velocity.
However, because the feed roller 7 and conveying roller 8 rotate in synchronization with one another as shown in FIG. 1, if the distance between the feed roller 7 and registration roller 8 is greater than the length of the least sheet size, the feed roller 7 unpreferably tends to feed a next sheet.
Further, the speed-changing mechanism 11 composed of clutches and gears is necessary in order to match the peripheral velocity of the conveying roller 8 with those of the feed roller 7 and registration roller 9. This would lower assembling workability resulting in sharp increase in cost. Besides, the driving clutches 10 and 15 should be adapted to operate substantially simultaneously with the speed-changing clutches 12 and 16, respectively. This would complicate combinations of operation-timing signals for controlling activation of the driving clutches 10 and 15 as well as the speed-changing clutches 12 and 16, thus markedly lowering reliability of the device.